Analog integrated circuits (IC's) such as analog-to-digital (A2D) and digital-to-analog (D2A) converters, for example, often require high precision capacitors to perform the analog to digital conversions. Some of the capacitor requirements for a true eighteen bit converter IC, for example, are a ratio stability of less than 0.00075% over 10 years, a voltage coefficient of less than 10 ppm per volt, a temperature drift match of less than 0.05% per degree Celsius, dielectric absorption of less than 0.00075% and capacitance greater than 0.5 fF per square micrometer, among other things.
Such integrated circuit capacitors are generally formed during the IC fabrication process whereby a thin dielectric layer is established between two conductive plates. A crucial limitation in manufacturing high precision integrated circuit capacitors is the formation of the capacitor dielectric. Because dielectrics composed of a single layer of dielectric material such as silicon dioxide or silicon nitride have unacceptably large voltage coefficients or dielectric adsorption, composite dielectric stacks of dielectrics with offsetting dielectric properties such as oxide/nitride/oxide dielectric stacks have been introduced to provide capacitor dielectrics with lower voltage coefficients and with acceptable dielectric absorption.
The voltage dependent capacitance, C(V), is given by the equationC(V)=1+βV+αV2 Where V is the voltage, β is the linear voltage coefficient of capacitance and α is the quadratic voltage coefficient of capacitance. Whereas β may be compensated by circuitry, α cannot. It is therefore desirable to form capacitor dielectrics with a low a in IC's that require high precision capacitors for such operations as A2D and D2A conversions.